Case studies

The scientific evidence of the extent to which agriculture impacts groundwater is herein examined by focusing on eight major river basins of Europe: Axios, Danube, Ebro, Oder, Pinios, Po, Seine and Vistula.
Collectively, these basins have a surface area of 1.4 million km², account for 14% of the EU territory and host a population of 157 million inhabitants (21% of the total).
The 85 studies considered for the analysis are well distributed across the basins and across Europe.

Overview

The extent of aquifer nitrification of Europe is unquestionable. The European Environment Agency estimates that the average nitrate concentration in groundwater aquifers of Europe is 18-19 mg NO₃/l (2000-2017; data from 16 member states).
In response to the Nitrates Directive, all Member States have designated Nitrate Vulnerable Zones and have established, or are in the process of establishing, comprehensive Programmes of Measures for the reduction of nitrate pollution of aquifers.
Four lines of evidence of agricultural impacts on groundwater were identified:

1. isotopic studies
2. aquifer water level decline
3. presence of pesticides, herbicides and phosphorus (direct impacts)
4. nitrate concentration in groundwater (indirect impact).

Isotopic studies

Isotopic studies provide information on hydrologic pathways, water mixing as well as the source of nitrates in groundwater.
Nitrate isotopes (¹⁵N-NO₃ and ¹⁸O-NO₃) and strontium isotopic ratios (⁸⁷Sr/⁸⁶Sr) have been used as geochemical fingerprints of nitrogen sources (manure, mineral fertilizers, sewage, and precipitation). The isotopic composition of water (²H/¹⁸O) and tritium measurements help identifying the proportion of recently infiltrated water versus old groundwater, as well as water residence time. Finally, carbon (¹⁴C-DIC) and sulphate (³⁴S) isotopes have been used to determine the age of water in the aquifers.
Isotopic studies indicate that the dominant sources of nitrate in aquifers are related to agricultural activities, particularly mineral fertilization and manure, followed by sewage and soil organic matter (Table 1).
In addition, most of the studies show that denitrification is a significant nitrate reduction pathway. The residence time of the water varies from a few to thousands of years, depending on the aquifer. These studies provide direct evidence linking agricultural activities to aquifer nitrate contamination across Europe.

Basin	Country	Source of nitrates	Transit time	Residence time	Denitrification	Reference
Danube	Austria	Manure, fertilizers		5-100 years	Not significant	Wild 2018
	Czechia	Fertilizers	21 months	22 years		Buzek 1998
	Czechia	Fertilizers	28 days			Buzek 2006
	Slovenia	Fertilizers, sewage, manure			Significant	Ogrinc 2019
	Serbia	Soil organic matter, sewage, manure			Significant	Miljevic 2012
Ebro	Spain	Fertilizers				Merchan 2014
Oder	Poland	Manure, fertilizers, sewage			Significant	Dragon 2016
	Poland		Upper aquifer - recent water	Deep 1000 years - circulation due to pumping		Dragon-Gorski 2015
Po	Italy	Fertilizers, manure			Significant	Bartoli 2012
	Italy	Fertilizers, manure, sewage			Significant	Lasagna 2019
	Italy	Fertilizers, manure, sewage			Significant	Martinelli 2018
	Italy	Fertilizers		251-11946 years		Pilla 2005
	Italy		Mean transit time 2-5 years	Recent infiltration		Rapti 2009
	Italy		Mixing of Oglio river with aquifer due to irrigation			Rotiroti 2019
	Italy	Mineral fertilizers, manure, sewage				Sacchi 2013
Seine	France	Fertilizers			Significant	Floury 2019
	France				Significant	Garnier 2014s
	France	Fertilizers			Significant	Sebilo 2003
Vistula	Poland	Fertilizers, sewage				Czekaj 2016

Aquifer water level decline

Water extraction for irrigation and domestic water supply at rates higher than the aquifer recharge causes continuous drop of the water table and depletes water resources (aquifer water level decline).
One of the worst situation was reported in the Pinios basin, where aquifer water level in some areas has dropped by hundreds of meters.
In areas close to the sea, declines of the water level can cause sea water intrusion. This has been observed in the deltas of Po and Pinios rivers, as well as in many coastal areas of the Mediterranean Sea, and is primarily caused by irrigation demand.
Large water extraction for irrigation in Murcia, Almeria and the Alto Guadalentin caused significant drop of aquifers water level, triggering land subsidence.
Small water level declines due to overextraction have been observed also in Austria, Hungary, Serbia, Romania and Italy. Such small water table declines can affect spring flow and impact sensitive ecosystems that depend on them, as in the case of the Po river basin in Italy.

Contamination by pesticides, herbicides and phosphorus

Studies on aquifers contamination by pesticides and herbicides in Axios, Ebro and Po Basins reported chemicals presence in groundwater.
In the Axios basin, contamination by pesticides (Atrazine and Prometryne) and herbicides (Alachlor, Propanil, Arbofuran and Paraoxon-methyl) was detected.
In the Ebro basin, Tributylphosphate (pesticide additive), Atrazine and Desethyl atrazine, DEA, Simazine, and DES were reported.
In the Po basin, a study conducted on five herbicides detected presence of Terbuthylazine (TBA) and its metabolite Desethylterbuthylazine (DET), likely originating from cropland. Perchlorate and its by-products (Chlorate and Chlorite), which are present in many fertilizers, were found in the shallow unconfined aquifer of Ferrara province, in the Po delta groundwater, indicating agricultural contamination.
Finally, ten studies reported phosphate, which is related primarily to agricultural activities (over-fertilization), in aquifers of Axios, Pinios, Oder and Vistula basins. Phosphate concentrations in Poland aquifers were very high, and have been attributed to excess fertilization.

Nitrate concentration in groundwater

The majority of scientific literature investigated nitrate concentration in groundwater.
Notwithstanding the large spatial and temporal variability of data, 68% of the studies reported maximum nitrate concentrations measured in aquifers exceeding drinking water standard limits (50 mg NO₃/l of nitrate). The presence of nitrate in groundwater is an indirect impact of diffuse pollution from agricultural land, as confirmed by isotope studies.

Conclusions

The protection of groundwater resources is pivotal in Europe, and several European Directives aim at their protection and restoration, such us the Water Framework Directive (WFD), The Nitrates Directive and the Groundwater Directive. The scientific literature gathered in the eight selected basins showed that:

• Methods for designation of Nitrate Vulnerable Zones should be improved to ensure effective identification of vulnerable areas;
• established Programmes of Measures for improving the water quality of aquifers too often lack definition of measurable actions to ensure gradual improvement. These actions should consider the hydrologic and geochemical response of impacted aquifers. In particular, aquifer residence time is often ignored, but this affect the time horizon required for restoration;
• in response to EU Directives, Member State report a wealth of monitored groundwater data, including both water quality and quantity. In general, scientific literature has not yet taken full advantage of these information sources. Conversely, monitored data are often documented in grey literature (e.g. national conference proceedings published in local languages), making it hard to retrieve and re-use data and hampering assessment of anthropogenic impacts on groundwater resources.

Fact sheets

References

Acknowledgement
The content of this web page is based on the scientific review prepared by Prof. Nikolaos Nikolaidis, Technical University of Crete, Greece